High fidelity sound translating apparatus



June 20', 1961 J. A. vlcTOREEN 2,989,597

HIGH FIDELITY souun TRANSLATING APPARATUS Filed June so, 1955 2 Shets-Shaet 1 INVENTOR. JOHN A. VICTOEEEN HIS ATTORNEY June 20, 1961 .1. A. VICTOREEN HIGH FIDELITY SOUND TRANSLATING APPARATUS Filed June 50, 1955 2 Sheets-Sheet 2 llllluallllll INVENTOR. JOHN A, V/CTOEEEN H/S ATTORNEY United States Patent 2,989,597 HIGH FIDELITY SOUND 'IRANSLATING APPARATUS John A. Victoreen, 512 Foothill Road, Colorado Springs, Colo. Filed June 30, 1955, Ser. No. 519,150 17 Claims. (Cl. 179-107) This invention relates primarily, although it is not necessarily limited to, improvements in hearing aids having wide range and high fidelity and to an improved microphone unit and preamplifier therefor.

Although hearing aids are well known to those versed in the art, there have been certain disadvantages in connection with the operation thereof such as the fact that they are relatively low fidelity devices, i.e., of the type which do not accept and reproduce sounds as they actually exist due to certain distortions that are inherent in the structures and which have in the past been ignored or accepted as inevitable to their operation.

By the hearing aid of the present invention I have provided one which is peculiarly immune to noise transients, and the generation of spurious sounds therein due to their operation, and wherein noise due to shock or mechanical vibration is substantially reduced. The apparatus adapts itself particularly to the use of binaural hearing aids although it is not limited thereto.

Among other advantages of the invention is an improved type of transducer which eliminates the effect of the inequalities of reproduction, such as sound response curve peaks due to resonants that are undesirable. The transducer also is particularly adapted for use in hearing aids having transistorized amplifiers and wherein the use of coupling transformers is completely eliminated with the elimination of their concomitant difficulties attendant upon the use of transformers.

In the drawings:'

FIG. 1 is a view of the hearing aid of my invention with the cover removed and with the alternate position of certain of the parts shown by dotted lines;

FIG. 2 is an end view taken from the output end and with the casing broken away to show the position of the microphone unit;

FIG. 3 is an edge view of the casing of FIG. 1 with the cover in position;

FIG. 4 is a diagrammatic view of a separate microphone unit;

FIG. 5 is a view taken on the line 5-5 of FIG. 4, looking in the direction of the arrow;

FIG. 6 is a schematic view of the microphone unit and a preamplifier therefor;

FIG. 7 is an elevational view of an embodiment as applied to a microphone unit; and

FIG. 8 is a plan view thereof.

Briefly the structure includes a casing in the form of a rectangular parallelepiped having sound receiving openings in one side and a transducer unit in back of the openings responsive substantially to sound coming only through the openings. As better shown in FIG. 1, I provide a casing 10 of generally rectangular formation having flanges 11 adapted to receive a cover 12, which is removably secured in the casing to provide a complete enclosure for the components.

Preferably the casing has a smooth and highly polished exterior which may be of metal such as brass plated with a hard substance such as chromium. This is contrary to the accepted practice in hearing aids in that decorative exteriors are normally supplied. By providing a highly polished hard, smooth exterior and which resists scratching or abrasion and remains smooth the movements of the hearing aid in contact with the wearer's clothing are Patented June 20, 1961 ice without the usual abrasive action which provides a highly undesirable background noise common to hearing aids.

At one edge of the casing and towards one end there are provided a plurality of openings 15 which extend from the exterior into the interior. The other portion of this edge of the casing is provided with a hinged cover 17 having a rotatable locking or slidable means 18. The cover carries a pair of U-shaped spring clips 19 insulated from the case. The cover may be opened by rotating the lock 18 for the insertion of or removal of the batteries 20. The connectors 19 provide a series connection between the batteries. p

A microphone unit is provided disposed interiorly of the casing and opposite to the openings 15. Basically the microphone comprises two or more transducers or microphones which are preferably of the crystal type, but are not necessarily limited to being crystal microphones, disposed in closely spaced parallel relation to each other with their sound sensitive faces opposing each other. The spacing between the microphones is such that the air column between the sound sensitive faces acts as a mutual loading between those faces. Preferably I use four of these microphones although it will be apparent that two could be used or even more and by their use accomplish several new and improved results among which may be included the following.

The frequency response of the device may be desirably altered by the adjustment of the spacing between the diaphragms so that either an essentially flat frequency response may be obtained, or, a desirable modification of a flat response may be obtained. In addition thereto, the proximity of the sound sensitive elements, which in this case may be diaphragms, although it might be the interfaces of the crystals themselves, or, other sound sensitive elements, are so close that actual interference between them, due to the air loading between them, may exist particularly at certain frequencies. In other words, if one sound sensitive element should have a tendency to resonate, vibrate or what is sometimes referred to as ring, which is a form of vibration excited by transient sound pulses, the effect is to cause the opposing diaphragm through the loading of the air column to mechanically, with very slight phase displacement, depending upon the frequency of the sound, follow the first diaphragm with a resultant damping" of the action on both diaphragms. Thus inequalities of vibration of one diaphragm relative to the other effects a supplemental vibration of the other diaphragm which offsets or cancels that of the first diaphragm. Furthermore, if these units are connected together properly or may electrically interact to operate each other the coupling results in an elimination of undesirable resonants or inequalities which may be considered as an averaging out of inequalities to provide an overall more uniform response.

A further advantage resides in the fact that the mounting, with the diaphragms in opposition, makes them uniquely immune to electric outputs arising from me chanical shocks. The use of more than one microphone is greatly advantageous in order to provide a simple averaging of the outputs of the individual elementsthus reduc ing' substantially distortion or resonance which may exist in any single unit. Having a plurality of microphones properly connected also provides a safety factor in that, should one of the microphones fail, the apparatus will still be operative to produce sound with reduced efliciency. When a plurality of microphones are used they may be arranged according to their individual characteristics to provide response curves having predetermined desired characteristics.

It should be pointed out that with the microphones operating in pairs itis possible by selection to provide a unit which has a desirable response curve. That is, it may be desirable to have a unit response which is greater in the high frequency end or one having greater response at some other point in the curve or to provide a sharper frequency response curve than that attainable in any ordinary way.

Although I have shown a specific form of structure for effecting the foregoing it should be understood that the illustrations are for the purpose of clarity and explanation of the invention rather than of limitation as it will be apparent to those versed in the art, with the disclosure, how many modifications could be made without departing from the spirit of the invention.

With the foregoing in mind I provide a frame consistof a pair of rectangular frame elements 30 each having flanges 31 and each frame having a pair of openings 3-2. The main bodies of the frame are provided with dimples 33 extending toward each other whereby the frames may be rigidly secured together by rivets 34. It is desirable that the frame be designed sufficiently rigid that it has no inherent resonant points of its own within the audible frequency range. I have found for instance that by providing a plurality of fastenings such as has just been described at approximately the four corner points and one between the central crossbars 36 in the center of the frame as shown at 37 the frame may be constructed sufliciently rigid to eliminate any vibration nodes of its own in the audible range.

The frame proper and the spacing between the diaphragms is determined by the depth of the dimples. It will be appreciated that the dimples only utilize restricted areas and that therefore the spacing between the frames around the edges is substantially completely open. Supported upon each of the frames is a pair of microphones. The drawings illustrate two such microphones carried by each frame side by side and facing toward two similar microphones carried on the other frame, thus there is a total of four microphones illustrated disposed in opposed pairs. The microphones may each comprise a housing 40 having a peripheral edge flange 41 and are disposed with their diaphragms opposite and adjacent the opening 32 in the frame. The flange of the microphone body may be secured to the frame by cementing or preferably by lugs 42 on the frame which are bent over into clamping relation with the edge flange on the microphone.

When the unit is disposed in a hearing aid case it is preferably disposed within the case with the open edges between the microphones opposite to the openings 15 and supported upon resilient sound absorbing and shock resistant material 43. There are many suitable materials available such as felt, silicon rubber, sponge rubber, etc. Prior to assembling the microphone unit in the housing, however, I surround the three edges of the opening with a sound barrier 44, closing the opening between the edges and leaving the only edges toward the opening 15 open. This sound barrier may be of any material that will prevent sound waves from getting into the space between the microphones and include a combination of a larger felt 45 and thin metal 44 as shown in FIGS. 1 and 2, and prevents possible case resonances from affecting the frequency response. Preferably the unit with its shock absorbing material should not fit into the case in such a manner that the shock absorbing material is greatly compressed. As a matter of fact it is believed desirable that the only compression upon the material be due to the gravitational forces upon the unit within the case. On the other hand, it should not fit so loose that appreciable motion results from the changing of the casing position due to normal body motions of the wearer. This has the effect that the mechanical shocks received by the casing normally are transmitted to the unit through one side only and are resisted by the mass of the microphone elements mechanically reacting through the resilient material.

Although I have shown what may be considered as a pompartment for the microphones consisting of the outer walls of the case and a partition 46 between the microphone compartment and the battery compartment it is within the concept of my invention that the partition 46 may be eliminated, particularly where the batteries are housed in a separate casing insertable within the casing as a unit, in which event the partition would not be needed. The amplifier chassis may contact with the shock absorbing material and furnish the other restraining wall for the microphone unit.

As can best be seen in FIG. 1, the pairs of microphones are separated from each other and sound may enter between them through the openings 15 and the open throat which is comprised of the two edge flanges and which leads to the interior space 52 between the two microphones. It may be desirable to place a piece of fibrous material 53 back of the openings 15 to prevent the ingress 'of dirt. I also contemplate that the throat may be filled with a pad of fibrous material although in many instances this may be eliminated. In addition I contemplate utiliz. ing a sheet of fibrous material 54 which may be suspended intermediate the microphones for sound damping purposes.

Although the microphone unit has been described and illustrated in FIGS. 1 to 3, inclusive as being used in combination with the hearing aid it will be apparent that the microphone, constructed according to the concepts of my invention, is exceedingly useful apart from the hearing aid as a general purpose high fidelity microphone.

Such a device is shown in FIGS. 4 and 5 which illustrate a unit wherein only two transducer elements are disposed in opposition and supported in frames in a manner similar to that previously described. The frames in this instance being spaced apart by spacers 56 and held together by screws 57 which pass through the flanges of the frames into the spacers. In this particular illustration it will be seen that the microphones are shown as being of the crystal type being supported at three corners by supports 58 connected to the housing 40; the fourth corner being coupled to an operating member 59 connected to the center of the diaphragm 60 which is dished inwardly toward the back of the housing and has the flanges secured to the exterior flange of the housing in a manner well known in the art.

It should be pointed out that microphones of various designs may be used, of which there are many on the market, those shown in the hearing aid being available and that the critical determinants are the spacing between the diaphragms, the manner in which the outputs are connected and in some instances the existency of a sound absorbing material in the throat or between the diaphragms.

I also contemplate that the microphones shown in FIGS. 4 and 5 may be used without the enclosed casing and shock absorbing material. FIG. 7 illustrates such a device which includes the pair of microphones 40' rigidly mounted together in spaced relation by pins 61 which are connected to a yoke 62 supported from a base 63. Preferably a small amount of dust inhibiting material and sound absorbing material 64 is placed between the openings of the microphone. It should be pointed out however that in this instance all sides are open and accept sound. Although I have previously shown the microphones as having square cases, round or other formations may be used as illustrated in FIGS. 7 and 8.

As previously stated the connection of the two microphones to each other in spaced relation is sufficiently rigid that if one of the casings moves in any direction the other moves along with it in the same direction. Therefore any movement which affects one casing will cause equal amounts of movements of the other casing in an exactly similar manner. The diaphragms however being supported in opposition, a shock other than a sound wave coming from the exterior causing the move rnent of one of the diaphragms will cause the movement s of the other diaphragm in the opposite direction in substantially equal amplitude and opposite phase. Thus, with their outputs properly connected, energy generatedby one of the sound translating elements, due to shock, will be cancelled by an opposite energy generated from the other sound translating element.

Additional advantages are obtained particularly when the two or more microphones are connected additively in parallel. Thus a motion of one diaphragm resulting from an impinging sound wave tends to drive the other diaphragm electrically as well as mechanically by the electric excitation of its driving element on the other driving element. The resulting load due to the mechanical effort of driving the second diaphragm and the air damping of this diaphragm reduces any resonant effects residual in either diaphragm. Thus any exaggerated frequency response at any particular frequency is reduced and electric output is uniform over the useful audio frequency range.

Certain advantages are obtained, as previously stated,- by connecting the outputs from the transducer units in parallel addition. Inasmuch as I have described the transducers as being crystals, that terminology will be maintained although it is very apparent that the outputs from such as dynamic and variable reluctance type could be utilized without departing from the invention. The effects described are most enhanced by types of microphones which may be electrically driven, to produce sound, as well as, producing electrical impulses from sound pressure changes, in which case, a maximum of interaction is obtained. This type of connection is shown in the diagram of FIG. 6 where it will be noted that the portions of like polarity are connected in parallel and the combined outputs are connected to a transistor preamplifier circuit.

Transistor circuits are particularly sensitive to the operating generator impedance as regards the production of noise in the circuit. Lowering the impedance of the microphone combination, permits that value of generator impedance to be attained at which the signal to noise ratio of the combination becomes maximum. In this instance the polarities are shown as positive and negative polarities which represent the polarity of the energy developed at one particular instant and would be reversed at some other instant of time in a transient sound wave. The outputs have one lead connected to the base of the transistor and the other lead through a resistor Rz to the emitter. The resistor R2 is shown as variable, this being the element which will be adjusted and changes the input impedance of the transistor input circuit. The value of Rz in combination with the impedance of the microphone is combined to produce the maximum signal to noise ratio. Once determined the resistance is fixed for a given type of microphone. Resistors R1 and R2 constitute a voltage divider providing base bias stabilization. The battery is shown connected through a resistor to the collector and thence through the resistor R1 and R2 to the base and emitter through resistance Rz respectively. Either R1 or R2 may be used as the output coupling resistor depending upon the position of switch sw, as may be desired, for the next amplifier stage. It should also be noted that the lower end of the transducer may be connected directly to the emitter, if desired, with some change in frequency response.

It should be noted that the greatest support for the microphone unit is directed toward the edges, being from the edge of the casing resulting in a material reduction in undesired sound due to shock and that preferably this support should be such that the microphone unit does not have its broad sides touch the case or anything connected with the case at any point. The amplifier circuitry may be an all transistor circuit, or a combination of transistors and vacuum tubes, or other amplifying means. The circuitry is supported by a pair of insulated strips or panels 70 and 71- which carry thereon the various circuitcomponents necessary to the formation of the aniplifier. Preferably, but not necessarily, each board carries approximately one-half section of the complete amplifier. It will be noted that the boards are also sup ported from the edge of the casing, the ends 70' and 71' being disposed in a notch at the upper end of the casing and h'avingdisposed therebetween in the same notch the output connector 72 for the earphone secured to the board 71 which carries the output portion of the amplifier. The casing at the other end may be provided with a boss 73 which supports the edges of the strips away from the sides of the casing. This construction provides an amplifier chassis which may be quickly and easily removed andinstalled thus making it easily accessible for servicing and which is also out of contact with the sides of the casing.

Of particular interest is the fact that the two strips are also held together by the male and female member connectors 74 and 75 in interlocking engagement on the inner faces of the strips and which members also serve to connect the two parts of the amplifier circuits electrically to each other. As will be noted the connector 74 at the lower end of the figure is rotated at right angles to that shown above which thus assists in maintaining alignments between the two parts when they are snapped together. Although only two connectors are shown it usually requires at least three or four to complete the circuit.

In a device of this character it is contemplated that in some instances it may be desirable that the device be preset for a user to a volume determined by suitable measurements made upon the persons hearing and that he be required to use it at this presetting for a certain length of time or even permanently. To this end the gain control is secured to the board 70 by any suitable means and the board 71 is provided with an opening 82' opposite to the squared shaft 81 of the gain control. A sleeve 83 is secured into the casing and extends against and surrounds the opening 82. This sleeve is provided with a tangential slot 84 and a spring clip 85 of the wellknown hair-pin type, or a C-ring, and is disposed around the sleeve with one end resiliently disposed in the slot. A control knob 86 having a stem 87 with a notch 88' is disposed within the sleeve with the notch in engagement with the squared end 81 for rotating the gain control. When in its innermost position the spring 85 drops into an annular groove 90 and retains the knob and shaft in operating position. However, should it be desired the knob may be removed by simply pulling the same out: ward and removing it from the case. Thus the control of the device or the lack of control can be retained at the option of the person dispensing the device or the user.

Having thus described my invention I am aware that numerous and extensive departures may be made therefrom without departing from the spirit or scope of the invention as defined by the appended claims.

I claim:

1. A microphone comprising a pair of electromechanical transducer units having respective movable sound responsive elements closely positioned in confronting relation opposite one another, a support for said transducer units, acoustic shielding material extending around the space between the confronting portions of the sound responsive elements of said transducer units, and means defining sound passage means leading into the space between the confronting portions of the sound responsive elements to pass sound thereto.

2. A microphone comprising a pair of electromechanical transducer units having respective movable sound responsive elements which present confronting faces closely positioned in parallel with respect to one another,- means connecting said transducer units electrically in parallel to generate voltages of the same polarity when they move in opposite directions, and a support for said 7 transducer units, acoustic shielding material extending around the space between said confronting faces of the sound responsive elements, said support having passag means leading into the space between said confronting faces of said sound responsive elements to pass sound thereto.

3. A microphone comprising a pair of electroacoustic transducer units having respective sound responsive elements closely positioned in confronting relation opposite one another, a casing enclosing said transducer units, acoustic shielding material extending around said units and continuously around the respective peripheries of said sound responsive elements throughout the major portion of said peripheries, and said casing having passage means at the remaining portion of the respective peripheries of the sound responsive elements leading into the space between said sound responsive elements.

4. A microphone comprising a pair of electroacoustic transducer units having respective vibratory sound responsive elements presenting faces which are closely positioned in confronting relation opposite one another, means connecting said transducer units electrically in parallel to generate voltages of the same polarity when said vibratory elements move in opposite directions, a casing enclosing said transducer units, acoustic shielding material extending continuously around the respective peripheries of said vibratory elements throughout the major portion of their extent and extending around the. portions of the transducer units away from said confronting faces of the vibratory elements, and said casing having passage means at the remaining portion of the respective peripheries of the vibratory elements which lead into the space between said confronting faces of the vibratory elements.

5. In a hearing aid, the combination of a casing having spaced front and back walls and edge walls interconnecting said front and back walls, a pair of electroacoustic transducer units completely enclosed Within said casing and having respective vibratory sound responsive elements closely positioned in confronting relation opposite one another and extending transverse to one of said edge walls of the casing in close proximity thereto, means connecting said transducer units electrically in parallel to produce voltages of the same polarity when they move in opposite directions and to produce voltages of opposite polarity when they move in the same direction, acoustic shielding material between the casing and said transducer units extending around the transducer units and completely around the space between the confronting portions of said vibratory elements except at said one edge wall of the casing, and said casing having passage means in said one edge wall leading into the space between the vibratory elements to pass sound thereto.

6. In a hearing aid, the combination of a casing having spaced front and back walls and edge walls interconnecting said front and back walls, a pair of piezoelectric transducer units completely enclosed within said casing and having respective vibratory sound responsive elements closely positioned in confronting relation opposite one another, said vibratory elements extending transverse to one of said edge walls of the casing in close proximity thereto, an amplifier enclosed within said casing and including a transistor having base, emitter and collector electrodes, a battery connected between the collector and emitter and a resistance between the base and emitter for biasing the emitter-base forwardly and resistance means between the collector and base for biasing the same rearwardly means connecting said transducer units electrically in parallel to produce voltages of the same polarity when their respective vibratory elements vibrate oppositely and to produce voltages of opposite polarity when their respective vibratory elements vibrate in phase, a direct connection from one pair of connected terminals inv said transducer units to said base and emitter electrodes of the transistor, acoustic shielding material .be-

tween the casing and said transducer units extending around the transducer units and extending continuously around the space between the vibratory elements except at said one edge wall of the casing, and said casing having passage means in said one edge wall leading into said space between the confronting faces of the vibratory elements.

7. In a hearing aid, the combination of a casing having spaced front and back walls and edge walls interconnecting said front and back walls, two pairs of piezoelectric crystal transducer units, means supporting said transducer units within the casing and with the pairs of units spaced apart from one another lengthwise along one of said edge walls of the casing, each pair of the transducer ,units having respective vibratory sound responsive elements closely positioned in confronting relation opposite one another, the vibratory elements of each pair of units extending transverse to said one edge wall of the casing in close proximity thereto, means connecting the transducer units of each pair electrically in parallel to produce voltages of the same polarity when their respective vibratory elements vibrate oppositely and to produce voltages of opposite polarity when their respective vibratory elements vibrate in phase, an amplifier within the casing including a transistor having base, emitter and collector electrodes, a battery connected between the collector and emitter and a resistance between the base and emitter for biasing the emitter-base forwardly and resistance means between the collector and base for biasing the same rearwardly a direct connection from one set of connected terminals in the transducer units to said base and emitter electrodes of the transistor, acoustic shielding material between the casing and the transducer units around the transducer units and around the respective spaces between the vibratory elements except at said one edge wall of the casing, and said casing having passage means in said one edge wall leading into the respective spaces between the vibratory elements of each pair of units.

8. A hearing aid comprising a casing having a pair of spaced major walls of broad surface area and edge walls interconnecting said major walls, said edge walls being of substantially smaller surface area than said major walls, one of said edge walls having passage means leading into the interior of the casing, a pair of interconnectcd rigid frames within the casing, a pair of electroacoustic transducer units mounted respectively on said frames and having respective vibratory sound responsive elements closely positioned in confronting relation opposite one another in close proximity to said passage means and at opposite sides of said passage means, and shock absorbing and acoustic shielding material between the frames and the casing and surrounding said transducer units except at said passage means.

9. The hearing aid of claim 8 further provided with an insulation panel supported at its opposite ends at oppositely disposed edge walls of the casing and lying substantially perpendicular to said major walls of the casing, said panel throughout its length being spaced from said major walls of the casing, and amplifier components mounted on said panel and coupled electrically to said transducer units.

10. A hearing aid comprising a casing having a pair of spaced major walls of broad surface area and edge walls interconnecting said major walls, said edge walls being of substantially smaller surface area than said major walls, a microphone enclosed within said casing, an insulation panel supported at its opposite ends at oppositely disposed edge walls of the casing and extending between said oppositely disposed edge walls transverse to said major walls, said panel throughout substantially its entire length between said oppositely disposed edge walls being spaced from said major walls of the casing, and a plurality of amplifier components mounted on said panel and coupled electrically to said microphone. I

11. A hearing aid comprising a rigid rectangular parallelepiped casing having front and back walls of broad surface area spaced from one another and edge walls interconnecting said front and back walls, said edge walls being of substantially smaller surface area than said front and back walls, a microphone within said casing, a pair of spaced substantially planar insulation panels mounted within the casing in closely positioned, substantially parallel relationship, each of said panels being mounted at its opposite ends in oppositely disposed edge walls of the casing and extending throughout its length in spaced relation to both said front and back walls of the casing, each of said panels extending in a plane substantially perpendicular to said front and back walls of the casing, and amplifier components mounted on said panels and coupled electrically to each other and to the microphone.

12. The hearing aid of claim 11, wherein there is provided an earphone socket exposed at one of said oppositely disposed edge walls of the casing and positioned thereat between said panels and spacing said panels apart.

13. A hearing aid comprising a rectangular parallelepiped casing having front and back walls of broad surface area and edge walls interconnecting said front and back walls, said edge walls being of substantially smaller surface area than said front and back walls, one of said edge walls having passage means leading into the interior of the casing, a pair of rigid frames said casing rigidly attached to one another and having openings which register with one another, a pair of electroacoustic transducer units mounted respectively on said frames and having respective vibratory sound responsive elements at said openings which are closely positioned in confronting relation opposite one another, said vibratory elements being positioned in close proximity to said passage means and at opposite sides of said passage means to define a space between themselves, resilient shock absorbing and acoustic shielding material between said frames and the casing supporting the frames resiliently within the casing and extending completely around said transducer units except at the confronting portions of said vibratory elements and at said passage means, and means connecting said transducer units electrically in parallel to produce voltages of the same polarity when their respective vibratory elements move in opposite directions and to produce voltages of opposite polarity when their respective vibratory elements move in the same direction.

14. The hearing aid of claim 13, further characterized by an insulation panel within the casing extending transverse to said front and back walls of the casing, said panel at its opposite ends being supported at oppositely disposed edge walls of the casing and extending therefrom in spaced relation to said front and back walls of the casing throughout substantially its entire length, and amplifier components mounted on said panel and coupled electrically to said transducer units.

15. The hearing aid of claim 14, in which each of said transducer units comprises a piezoelectric transducer, one of said amplifier components is a transistor having base, emitter and collector electrodes, a battery connected between the collector and emitter and a resistance between the base and emitter for biasing the emitter-base forwardly and resistance means between the collector and base for biasing the same rearwardly and one connected pair of terminals of said transducer units is connected directly to said base and emitter electrodes of the transistor.

16. In a hearing aid, the combination of a casing, a

pair of piezoelectric transducer units completely enclosed within said casing and having respective vibratory sound responsive elements closely positioned in confronting relation opposite one another, an amplifier enclosed Within said casing and including a transistor having base, emitter and collector electrodes, a battery connected between the collector and emitter and a resistance between the base and emitter for biasing the emitter-base forwardly and resistance means between the collector and base for biasing the same rearwardly means connecting said transducer units electrically in parallel to produce voltages of the same polarity when their respective vibratory elements vibrate oppositely and to produce voltages of opposite polarity when their respective vibratory elements vibrate in phase, a direct connection from one pair of connected terminals in said transducer units to said base and emitter electrodes of the transistor, acoustic shielding material between the casing and said transducer units extending'around the space between the vibratory elements, and said casing having passage means leading into said space between the confronting faces of the vibratory elements to pass sound thereto.

17. A hearing aid comprising a casing having a pair of spaced major walls of broad surface area and edge walls interconnecting said major walls, said edge walls being of substantially smaller surface area than said major walls, one of said edge walls having passage means leading into the interior of the casing, a pair of electroacoustic transducer units mounted within the casing and having respective vibratory sound responsive elements closely positioned in confronting relation opposite one another in close proximity to said passage means and at opposite sides of said passage means, and acoustic shielding material within the casing surrounding said transducer units except at said passage means.

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